MAC/do: Remove PR_METHOD_REMOVE method

It isn't really needed, since common jail code destroys jail OSD storage
at jail destruction (via osd_jail_exit()), triggering our destructor
dealloc_osd(). Leveraging this mechanism is arguably even better as it
causes deallocation to always happen without the 'allprison_lock' lock.

While here, make the static definition of 'methods' top-level, renaming
it to 'osd_methods'.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47599